Interpretive Summary: In response to elevated nutrient intake by prepubertal Holstein heifers, mammary parenchyma DNA was reduced and fat pad DNA was increased when compared to restricted intake-controls at a common BW, but different ages. Most of the observed variation in parenchyma DNA was due to the difference in age at slaughter rather than level of nutrient intake. Furthermore, elevated nutrient intake did not influence lipid or protein partitioning to the parenchyma but did increase lipid deposition in the fat pad. Taken together, these data suggest that while the mammary fat pad is responsive to plane of nutrition, the parenchyma is not. These observations call into question the long held belief that nutrient status directly influences parenchyma development in the prepubertal heifer.

Technical Abstract: Elevated nutrient intake prior to puberty has been shown to retard prepubertal mammary development in the heifer. Our objective was to evaluate the effects of increased nutrient intake on mammary development in Holstein heifers at multiple body weights from birth through puberty. Specifically, we evaluated effects of nutrient intake and body weight at slaughter on 1) total weight and DNA content of the parenchyma and fat pad and 2) parenchyma and fat pad composition. Starting at 45 kg, heifers (n = 78) were assigned to either a restricted (R) or elevated (E) plain of nutrition supporting 650 (R) or 950 (E) g/d. Heifers were slaughtered at 50 kg increments from 100 to 350 kg. Fat pad weight and DNA content was greater in E- than R-heifers. Additionally, E-heifers had a greater fraction of lipid and a smaller fraction of protein in their mammary fat pad than R-heifers. Parenchyma weight and DNA was significantly lower in E- than R-heifers, however, when analyzed with age as a covariate term, plane of nutrition was no longer a significant term in the model. Plane of nutrition and daily growth rate had no effect on the lipid, protein, or hydroxyproline composition of the parenchyma. Collectively, these data demonstrate that the mammary parenchyma is refractory to level of nutrient intake while the mammary fat pad is not. Furthermore, covariate analysis demonstrates that age at slaughter, not level of nutrient intake, was the single greatest determinant of total parenchyma DNA content.